Future wireless systems require a more effective utilization of the radio frequency spectrum in order to increase the data rate achievable within a given transmission bandwidth. This can be accomplished by employing multiple transmit and receive antennas combined with signal processing. A number of recently developed techniques and emerging standards are based on employing multiple antennas at a base station and the mobile. Some of these can be used to improve the reliability of data communication over wireless media without compromising the effective data rate of the wireless systems. So called space-time codes (STCs) are used to this end. Some are used to increase the data rate of the wireless system. Systems such as Bit Interleaved Coded Modulation (BICM) in a Multiple Input Multiple Output (MIMO) scenario have been used for this purpose. Furthermore, recent advances in wireless communications have demonstrated that by jointly encoding symbols over time and transmit antennas at a base station one can obtain reliability (diversity) benefits as well as increases in the effective data rate from the base station to each cellular user. BICM, or BICM combined with STCs, provide such tradeoffs.
The multiplexing gains and diversity benefits are also inherently dependent on the number of transmit and receive antennas in the system being deployed, in the sense that they are fundamentally limited by the multiplexing-diversity trade-offs curves that are dictated by the number of transmit and the number of receive antennas in the system.
When a group of transmit antennas are used to send a single transmission containing information for a single user to a single receiver, which may also have multiple receive antennas, the resulting class of systems is typically referred to as single-user (SU)-MIMO systems.
A number of systems have been proposed for SU-MIMO-based transmission. Most state-of-the-art schemes rely on providing high data rates via wideband transmission that relies on the use of OFDM, since OFDM makes an equalizer unnecessary. With multilevel modems, coded modulation systems can easily be designed by use of an outer binary convolutional code and an interleaver in a BICM system. Most state-of-the art systems employ coded OFDM/MIMO-based transmission of this (or similar of a) form, whereby each groups of time/frequency slots are mapped into resource blocks and multiple users compete for scheduling in each resource block. In general, once a user is scheduled, coding occurs, often using an outer binary code with rate Rc, followed by an interleaver and a mapper that maps groups of bits to complex valued symbols, adhering to, e.g. a Quadrature Amplitute Modulation (QAM) constellation of size Q. These symbols are passed in round robin fashion to the antennas for OFDM transmission over the appropriate resource block. Typically, the users are scheduled for transmission by looking at a channel quality level indicator (CQI), such as their nominal received signal level. Although such aggregate CQI values can prove accurate indicators of the achievable rates in single-input single-output (SISO) transmission, they are a lot less meaningful for MIMO transmission. Indeed, two different MIMO channels with the same CQI level could support drastically different rates.
In many of the existing and emerging wireless system standards supporting SU-MIMO, within any “resource block channel,” a scheduler and/or rate adaptation mechanism decides which receiver (user) to serve and chooses a rate to transmit to this user. The rate is selected from one a number of possible transmission rates often based on scheduling criteria and/or a user's channel state between the transmitter and receiver. A rate is supported by a single transmission mode, each of which could be implemented by a specific STC, or BICM system with a given outer code rate and constellation, or combination. Given the scheduler's decision, point-to-point SU-MIMO transmission supporting this rate is then used from the transmitter to that receiver.
It is important to note that a given transmission rate (mode) can be supported at the receiver equipment by a number of possible different receiver algorithm designs. However, existing scheduler and/or rate adaptation mechanisms do not consider the particular receiver algorithm implementation or design, or the fact that receiver equipment may have more than one design at its disposal for a given rate (mode).
It is well known that SU-MIMO schemes work well at high rates for rich scattering channels, i.e., for channels where several parallel streams are created using multiple transmit antennas. On the other hand, with a channel with very strong direct paths but limited scattering, a much lower rate can be typically supported. It is therefore important that the system rate is well matched to the channel's ability to support a certain rate. If not, an outage event will inevitably occur, if too high transmission rate is attempted over the channel.
Here it is important to note a given transmission rate can be supported by a number of different transmission options, e.g. different BICM and/or STC combinations can result in the same transmission rate. These can behave differently depending on the channel and receiver algorithm being used.